Illumination control system



April 15'; 1969 Filed Oct. 11, '1966 J- J. SCHOEN ILLUMINATfON CONTROL SYSTEM INVENTOR. JOHN J. SCHOEN A TTOR/VEYS April 15, 1969 SCHQEN 3,438,704

ILLUMINATION CONTROL SYSTEM Sheet INVENTOR. JOHN J. SCHOEN ATTORNEYS Filed Oct 11; 1966 .I- J. SCHOEN ILLUMINATION CONTROL SYSTEM April 15; 1969 Sheet FildQct ..fl 1, 1966 FIG. 3

INVENTOR JOHN JQZZHOEN FIG. 5

A TTORNEYS April 1969 J. J. SCHOEN ILLUMINATION CONTROL SYSTEM Sheet 4 of 4 md on. 11. *1966 FIG. 7

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A TTORNEYS I United States Patent Office 3,438,704 Patented Apr. 15, 1969 3,438,704 ILLUMINATION CONTROL SYSTEM John J. Schoen, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 11, 1966, Ser. No. 585,908 Int. Cl. G03g /08 US. Cl. 355-8 8 Claims This invention relates in general to electrostatic recording and, in particular, to an optical system for projectinga line scan image onto a moving photoreceptor, such as, a xerographic plate, and an illumination control therefor.

In the practice of xerography, as described for example in US. Patent No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material afiixed to a conductive backing is used to support electrostatic images. In. the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image beingrepro duced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a powder. The powder is held in image areas by the electrostatic charge on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably aflixed thereto to form a permanent print.

It can be readily appreciated that the quality of the print is, in the large part, dependent on the exposure of the charged xerographic plate to the radiation image. The largest single factor effecting exposure latitude, i.e., range of illumination intensity, is the efliciency of the developer system. In other words if the developer system is highly sensitive so as to develop background or image portions as grey areas when in reality these are white, then illumination control must be commensurately sensitive to provide the proper exposure of the charged xerographic surface. With modern day improvements to xerographic developer systems, such as, an electrically biased backing electrode to enhance solid area development, the desirability of maintaining proper illumination becomes increasingly apparent.

A uniformly high level of illumination as required for exacting exposure is complicated by many factors, For example, variation in lamp output due to changes in ambient temperature or even age alone is sufficient to cause development of white areas thereby detracting from the overall quality of the print. Moreover, such things as dust on the platen and a high density document can affect the illumination level resulting in white image portions being developed as grey.

Now in accordance with the present invention the above drawbacks are overcome by an automatic illumination control to maintain illumination at a preselected level. More than this the illumination control is a linear system greatly simplifying the operation and maintaining fixed tolerances throughout the range of operation. Also the quality of the image is not deleteriously affected as is the case in an iris shutter mechanism commonly used as an illumination control on cameras.

It is therefore an object of this invention to improve illumination used in high speed automatic reproduction equipment capable of producing high quality copies.

Another object of this invention is to control the intensity of a printing light beam impinging on the surface of a photoreceptor at a predetermined level.

A further object of the invention is to compensate for changes in illumination in an optical system more simply and rapidly than heretofore.

A still further object of the invention is to provide a novel automatic illumination control system involving relatively few moving parts and which is reliable in operation.

The above objects as well as other features and advantages of the invention are accomplished, generally speaking, by positioning a variable aperture slit in a scan type optical system containing an optical lens primarily corrected for aberrations resulting from tangential optical ray-s. The aperture slit is formed by opaque wall members arranged for movement toward and away from the central path of the imaging rays in response to a predetermined change from a reference illumination level that is established by a control circuit operating in conjunction with a servo control to position the wall members.

For a better understanding of the invention reference is now had to the following detailed description of the invention to be read in connection with the accompanying drawings; wherein:

FIG. 1 illustrates schematically a preferred embodiment of a xerographic recording apparatus having a line scan type optical system with illumination control according to the present invention;

FIG. 2 is an isometric view of the optical system;

FIG. 3 is a side elevational view of the slit aperture mechanism;

FIG. 4 is a cross-sectional view of the slit aperture mechanism taken along lines 44 of FIGS;

FIG. 5 and FIG. 6 are left-hand and right-hand end views of the aperture slit mechanism, respectively;

' FIG. 7 is a diagrammatic view of a bridge circuit il lustrating the relationship of the active and reference sensing devices; and,

FIG. 8 is a wiring diagram of the control circuit for automatically controlling the illumination at the surface of the xerographic plate.

Referring now to FIG. 1 there is shown schematically a preferred high speed automatic xerographic reproduction machine incorporating optical scanning appanatus which includes an automatic illumination control according to the present invention whereby high quality reproduction is accomplished.

The automatic xerographic reproducing apparatus described more fully in US. Patent 3,221,622, comprises a xerographic plate 20 including a photoconductive layer on a conductive backing and formed in the shape of a drum, which is mounted on a shaft journaled in :a frame to rotate in the direction indicated by the arrow to cause the drum surface sequentially to pass a plurality of xerographic processing stations.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally, as follows:

A charging station at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station which a light or radiation pattern of copy to be reproduced is projected ontothe drum surface to dissipate the drum charge the exposed areas thereof and thereby form a latent electrostatic image of the copy to be reproduced;

A developing station at which a xerognaphic developing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image are cascaded over the drum surface, whereby the toner particles adhere to the electrostatic latent image to form a xerographic powder image in the configuration of the copy being reproduced;

A transfer station at which the xerographic powder image is electrostatically transferred from the drum surface to a transfer material or support surface; and,

A drum cleaning and discharge station at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer, and at which the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

The charging station is preferably located, as indicated by reference character A. As shown, the charging arrangement includes a corona charging device 21 which includes a corona discharge array of one or more corona discharge array of one or more corona discharge electrodes that extend transversely across the drum surface and are energized from a high potential source and are substantially closed within -a shielding member.

Next subsequent thereto in the path of motion of the xerographic drum is an exposure station B. In accordance with the present invention illumination control, to be described more fully hereinafter, is accomplished by selectively opening and closing an aperture slit mechanism 22 positioned in the path of a flowing image projected from a stationary original onto the surface of drum The optical scanning or projection asembly comprises a setationary oopyboard which consists of a transparent curved platen member 25 such as, for example, a glass plate or the like, positioned on the exterior of the cabinet, which is adapted to support a document to be reproduced, the document being uniformly illuminated and arranged in light projecting relation to the moving light-receiving surface of the xerographic drum. Uniform lighting is provided by bands of lamps arranged on opposite sides of the copyboard. Scanning of the document on the stationary oopyboard is accomplished by means of a rnirror assembly which is oscilliated relative to the copyboard in time-d relation to the movement of the xerographic drum.

The mirror assembly, which includes an object mirror 31, is mounted below the copyholder to reflect an image of the document through a spherical lens 33 onto an image mirror 35 which, in turn, reflects the image onto the xerographic drum.

Adjacent to the exposure station is a development station C in which there is positioned a developer apparatus 38 including a casing or housing having a lower or sump portion for accumulating developer material. A bucket conveyor (not shown) is used to carry the developing material to the upper part of the developer housing where it is cascaded over a hopper chute onto the xerographic drum to effect development.

Positioned next and adjacent to the developing station is the image transfer station D which includes a sheet feeding arrangement adapted to feed sheets of support material, such as paper or the like, successively to the xerographic drum in coordination with the presentation of the developed image on the drum surface at the transfer station.

The sheet feeding mechanism inicludes a sheet feed device 40 adapted by means of vacuum feeders to feed the top sheet, of a stack of sheets to roller 42 cooperating with the belts of paper transport 44 for advancing the sheet sufficiently to be held by paper transport 44 which in turn, conveys the sheet to a sheet registration device 45 positioned adjacent to the xerographic drum. The sheet registration device arrests and aligns each individual sheet of material and then in timed relation to the movement of the xerographic drum, advances the sheet material into contact with the xerographic drum in registration with a previously formed xerographic powder image on the drum.

The transfer of the xerographic powder image from the drum surface to the sheets of support material is effected by means of a corona transfer device 51 that is located at or immediately after the line of contact between the support material and the rotating drum. In operation, the electrostatic field created by the corona transfer device is effective to tack the support material electrostatically to the drum surface, whereby the support material moves synchronously with the drum while in contact therewith. Simultaneously with the tacking action, the electrostatic field is effective to attract the toner particles comprising the xerographic powder image from the drum surface and cause them to adhere electrostatically to the surface of the support material.

Immediately subsequent to the image transfer station, there is positioned a stripping apparatus to paper pick-off mechanism 52 for removing the sheets of support material from the drum surface. This device, which is of the type disclosed in Rutkus et a1. United States Patent 3,062,536, includes a plurality of small diameter orifices supplied with pressurized aeriform fluid by a suitable pulsator or other device. The pulsator is adapted to force jets of pressurized aeriform fluid through the outlet orifices into contact with the surface of the xerographic drum slightly in advance of the sheet of support material to strip the leading edge of the sheet from the drum surface and to direct it onto an endless conveyor 55 whereby the sheet material is carried to a fixing device 60. At the fixing device, the transferred xerographic powder image on the sheet of support material is permanently fixed or fused thereto as by heat. After fusing, the reproduction is dis charged from the apparatus at a suitable point for collection externally of the apparatus by means of the conveyor 65.

The next and final station in the device is a drum cleaning station E, having positioned therein a corona preclean device 76, a drum cleaning device adapted to remove any powder remaining on the xerographic drum after transfer by means of a rotating brush 81, and a discharge lamp-83 adapted to flood the xerographic drum with light to cause dissipation of any residual electrical charge remaining on the xerographic drum.

It is believed that the foregoing description is sufficient for the purposes of this application to show the general operation of a xerographic reproducing apparatus. For further details concerning the specific construction of the xerographic apparatus reference is made to U.S. Patent 3,221,622.

Refering now to FIGURES 2-6, there is illustrated in greater detail the automatic illumination control system according to the present invention. The intensity of illumination is regulated by the opening in slit aperture mechanism 22 interposed in the optical path adjacent the lens as best shown in FIG. 2. The illumination level at the surface of the drum is detected and compared with a reference illumination level in a control circuit described hereinafter which feeds back a signal to the slit aperture mechanism to maintain illumination approximately equal to the preset reference illumination level.

Slit aperture mechanism 22 is positioned so that elongated slit opening 101 has its longitudinal axis transverse to the direction of scan. By this arrangement tangential light rays pass parallel to the slit opening 101 while sagittal light rays pass normal to it. For this reason spherical lens 33 is corrected for oblique spherical abberation in the tangential plane only. The construction of such a lens is described more fully in copending application Ser. No. 585,936 of David C. Harper, filed Oct. 11, 1966. It should be noted that slit aperture mechanism 22 may be located on either the image side of lens 23 as illustrated on the object side so long as the slit opening is closed spaced to the lens so as not to deleteriously affect the quality of the image.

Slit aperture mechanism 22 has a variable slit in a direction normal to the optical path to thereby increase or decrease the illumination received at the drum and comprises a housing in which flexible wall members 112 and 114 are arranged so as to be movably slidable therein.

Wall members 112 and 114 are impervious to light and made from any suitable material, such as, Mylar dyed black. Rotatably journaled in the sides of housing 110 are a pair of shafts 122 and 124. On each end of shafts 122 and 124 are mounted sprocket members 128 which engage apertures 130 formed in wall members 112 and 114.

To move the flexible wall members in unison there is provided a reversible drive motor 135 which drivingly engages counter-rotating spur gears 141 and 143 which in turn drive gears 145 and 146 mounted on the ends of drive shafts 122 and 124, respectively. Also mounted on the shafts 122 and 124 are a pair of cams 148 and 149 which serve to actuate limit switches 150 and 151, respectively, located in the rotative path of the cams.

A light shield 166 (see FIG. 2) in the form of an open elongated box is positioned in close proximity to the peripheral surface of the drum '20. Formed. in the light shield is a narrow slot 167 extending substantially across the length thereof with the center line of the slot parallel to the axis of rotation of the drum.

Light shield 166 houses a photocell 170 and an associated lamp 172 in the interior thereof outside of the image area which serve as a part of the control circuit. In compartment 175 outside of the optical path is another photocell 180 and an associated lamp 182 which together also form a part of the control circuit.

Photocells 170 and 180 serve as leg components in a sensing bridge 185 shown in FIG. 7. The photocells can be made from any suitable material, such as, cadmium sulphide. Photocell 170 serves as the active cell in the optical illumination path and is positioned to receive il lumination reflected from a reference strip 190 on the platen best seen in FIG. 2. Reference strip 190 is colored to achieve a reference reflectance, such as, white. Photocell 180 is used as a reference in the bridge circuit for comparison with the illumination received by photocell 170. If the illumination received by photocell 170 and 180 is the same there is a balanced condition in the bridge and no current flows. However, if the illumination differs current will flow in the bridge which is coupled electrically to motor 135 as will become apparent. Initially sensing bridge 185 is set for a predetermined light intensity. This is accomplished by changing the resistance in variable resistors 210 and 212. Alternatively the intensity of lamps 172 and 182 can be varied to balance the bridge. Upon depressing the print button on the control panel (not shown) lamp 172 is dc-energized enabling the preset value of illumination to be compared with the illumination actually received from the optical system. Lamp 172 is energized when the machine is off to maintain the speed of response of photocell 170. Hence when the machine is turned on rapid response is attained. Current flows when the bridge is unbalanced to provide a polarized signal to motor 135 causing it to run forward or backward according to whether more or less illumination is required. When the proper illumination is present, the bridge is once more balanced and motor 135 is dc-ener-gized and braked, as explained hereinafter.

The operation of the illumination control may best be understood in connection with the circuit diagram in FIG. 8. The control circuit for actuating motor 135 is coupled to terminals W, X, Y, and Z of the sensing bridge as shown in FIG. 8. When there is an unbalanced condition in the bridge a differential amplifer 215 provides a signal to a pair of class -B amplifiers 220 and 225 connected in push-pull, i.e., when one is on the other is off. The output signal from the class B amplifiers operates motor 135.

The output signal from the sensing bridge 185 is applied to the bases of transistors Q1 and Q2 which com-prise differential amplifier 215. The polarity of the signal produced by the differential amplifier depends upon whether the illumination is higher than or less than the preset value. In the case where it is greater than the preset value, the voltage would be higher. On the other hand,

when the light impinging on photocell 170 is less than the preset value there is a lower voltage. The voltage change must exceed a certain dead band to operate class B amplifiers 220 and 225 which are normally in the off condition. This dead band is determined by the biasing of transistors Q3 and Q4.

Q3 is an NPN transitor and is turned on when receiving a positive signal at its base. In similar fashion,

Q4 is a PNP transitor responsive to a negative signal exceeding the dead band turning the transitor in the on condition. When Q3 is turned on this causes transitors Q5 and Q6 to conduct. Q5 and Q6 serve to amplify and invert the signal, respectively. The signal output from Q5 turns on power switch Q7 to provide a positive voltage level with respect to ground across the terminals of motor driving wall members 112 and 114 further apart to increase the illumination on the drum. In like fashion Q4 will turn on Q8 which further amplifies the signal rendering Q9 conductive. When Q9 conducts a negative voltage level is supplied across the terminals of the motor to drive the wall members closer together thereby decreasing illumination available at the surface of the drum.

' To provide a quick stop for wall members 112 and 114 a breaking resistor 250 is connected across the armature terminals of motor 135. Braking resistor 250 provides a low impedance to ground to dissipate energy stored in the motor. Limit switches and 151 prevent the wall members from closing completely or opening too far apart as previously discussed.

Above is described an illumination control system wherein an image projected onto a moving photoreceptor has a predetermined illumination level irrespective of ambient conditions, age of lamps and document density. Hence when a relatively dense document is to be reproduced, the exposure may be modified by greater illumination on the photorecetor as, for example, a xerographic drum. As can readily be appreciated such a system is highly desirable for Xerographic machines havmg sensitive development systems.

While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed is:

1. In a recording apparatus of the type in which a document to be reproduced in projected in line scan fashion onto a photoreceptor moved past an exposure station characterized by an illumination control system comprising in combination:

support means located at the object plane to position a document to be reproduced,

a source of illumination optically positioned to illuminate the document,

a photoreceptor at the image plane to receive a pattern of light and shadow according to the document to be recorded,

an optical system to project a line scan image of the document onto said photoreceptor, said optical system including a spherical lens corrected for oblique spherical aberration in the tangential plane,

light masking means interposed in the optical path in close proximity to said lens including a pair of spaced apart wall members impervious to light adapted for movement transverse to the optical center line in the sagittal direction to mask out selected light rays,

drive means operative to position said wall members toward and away from the optical center line to control the intensity of illumination at the surface of said photoreceptor at a predetermined level in response to discrete bi-polar electrical signals,

circuit means coupled to said drive means to emit discrete bi-polar electrical signals to actuate said drive means to move said wall members toward and away from the optical center line in a sagittal direction to maintain a predetermined level of an illumination on the surface of said photoreceptor with a minimum of optical deterioration thereat.

2. Apparatus according to claim 1 wherein said circuit means includes a sensing bridge having a pair of photocells coupled in opposing circuit paths, a one of said photocells being positioned in the optical path outside of the image area, the other of said photocells being positioned to receive a predetermined level of illumination from a reference source of illumination located exterior to the optical path.

3. Apparatus according to claim 2 wherein said drive means includes a reversible motor responsive to bi-polar signals received from said circuit means, said motor being drivingly connected to a pair of shafts mounted for counterrotation, said shafts being arranged to extend transverse to the optical path in parallel relation in the tangential plane on opposite sides of the optical center line, and sprocket members mounted on said shafts to engage apertures formed in said wall members for exerting bi-directional movement thereto upon actuation of said motor.

4. Apparatus according to claim 3 wherein said motor has a low impedance across the terminals of its armature whereby stored energy is rapidly dissipated from said motor to effect dynamic braking of said drive means.

5. Recording apparatus for controlling illumination at a light receiving surface arranged to receive a flowing image of a document to be recorded comprising:

a curved copyboard positioned at the object plane for supporting a document to be reproduced,

a first source of illumination arranged to illuminate the document,

a xerographic plate at the image plane adapted for movement to receive a flowing pattern of light and shadow according to the document to be recorded,

an optical system including a mirror adapted to be oscillated to project a flowing image of the document onto said xerographic plate in synchronization with the speed thereof, and a spherical lens corrected for oblique sphrical aberration in the tangential plane,

light masking means interposed in the optical path in close proximity to said lens including a pair of spaced apart wall members impervious to light adapted for movement transverse to the optical center line in the sagittal direction to mask out selected light rays,

first photosensitive means interposed in the optical path outside of the image area adjacent the surface of said xerographic plate,

second photosensitive means positioned exterior to the optical path,

a second source of illumination in close proximity to said second photosensitive means for directing light of predetermined intensity toward said second photosensitive means,

motor driven drive means operative to position said wall members to control the intensity illumination at the surface of said xerographic plate at a predetermined level in response to discrete bi-polar electrical signals,

circuit means including a sensing bridge coupling said first and second photosensitive means in opposed current paths across positive and negative voltage terminals, a dilferential amplifier connected to said sensing bridge to establish a polarized output signal, a pair of driver amplifiers connected in push-pull to receive said polarized output signal from said differential amplifier, and a motor armature to receive the output from said driver amplifiers to actuate said motor driven drive means in a direction according to the polarity of said output signal to selectively position said wall members to maintain illumination at a predetermined level of intensity at the surface of said xerographic plate.

6 Apparatus according to claim wherein said first photosensitive means is optically aligned with a strip located on said copyboard adjacent the document and arranged to be projected therewith by said optical system, said strip being colored for selective reflectance in the direction of said first photosensitive means.

7. Apparatus according to claim 5 wherein said circuit :means includes a resistor connected across the terminals of said motor armature to effect dynamic braking of said motor driven drive means.

8. Apparatus according to claim 5 wherein said xerographic plate is a drum mounted for rotating to pass a plurality of xerographic processing stations radially displaced from the drum axis for recording continuously.

References Cited UNITED STATES PATENTS 2,161,371 6/1939 Mees 88-24 3,062,110 11/1962 Shepardson et al. 88-24 5 3,330,180 7/1967 Ferguson et al. 88-24 3,379,107 4/1968 Lieser 95-l0 3,380,339 4/1968 Moss 8824 NORTON ANSHER, Primary Examiner. W. A. SIVERTSON, Assistant Examiner.

US. Cl. X.R. 355-68 

1. IN A RECORDING APPARATUS OF THE TYPE IN WHICH A DOCUMENT TO BE REPRODUCED IN PROJECTED IN LINE SCAN FASHION ONTO A PHOTORECEPTOR MOVED PAST AN EXPOSURE STATION CHARACTERIZED BY AN ILLUMINATION CONTROL SYSTEM COMPRISING IN COMBINATION: SUPPORT MEANS LOCATED AT THE OBJECT PLANE TO POSITION A DOCUMENT TO BE REPRODUCED, A SOURCE OF ILLUMINATION OPTICALLY POSITIONED TO ILUMINATE THE DOCUMENT, A PHOTORECEPTOR AT THE IMAGE PLANE TO RECEIVE A PATTERN OF LIGHT AND SHADOW ACCORDING TO THE DOCUMENT TO BE RECORDED, AN OPTICAL SYSTEM TO PROJECT A LINE SCAN IMAGE OF THE DOCUMENT ONTO SAID PHOTORECEPTOR, SAID OPTICAL SYSTEM INCLUDING A SPHERICAL LENS CORRECTED FOR OBLIQUE SPHERICAL ABERRATION IN THE TANGENTIAL PLANE, LIGHT MASKING MEANS INTERPOSED IN THE OPTICAL PATH IN CLOSE PROXIMITY TO SAID LENS INCLUDING A PAIR OF SPACED APART WALL MEMBERS IMPREVIOUS TO LIGHT ADAPTED FOR MOVEMENT TRANSVERSE TO THEE OPTICAL CENTER LINE IN THE SAGITTAL DIRECTION TO MASK OUT SELECTED LIGHT RAYS, DRIVE MEANS OPERATIVE TO POSITION SAID WALL MEMBERS TOWARD AND AWAY FROM THE OPTICAL CENTER LINE TO CONTROL THE INTENSITY OF ILLUMINATION AT THE SURFACE OF SAID PHOTORECEPTOR AT A PREDETERMINED LEVEL IN RESPONSE TO DISCRETE BI-POLAR ELECTRICAL SIGNALS, CIRCUIT MEANS COUPLED TO SAID DRIVE MEANS TO EMIT DISCRETE BI-POLAR ELECTRICAL SIGNALS TO ACTUATE SAID DRIVE MEANS TO MOVE SAID WALL MEMBERS TOWARD AND AWAY FROM THE OPTICAL CENTER LINE IN A SAGITTAL DIRECTION TO MAINTAIN A PREDETERMINED LEVEL OF AN ILLUMINATION ON THE SURFACE OF SAID PHOTORECEPTOR WITH A MINIMUM OF OPTICAL DETERIORATION THEREAT. 